1. Field of the Invention
The present invention generally relates to a color printer for printing a full color image with plural thermal heads, and further relates to preventing the shear of the colors in printing, such as errors in dot positioning. Also, the present invention generally relates to paper feeding of color printers with plural thermal heads so as to prevent shear of the colors in printing.
Also, the invention generally relates to thermal heads, which are very useful for the above mentioned color printers, and which can be used in double-line thermal heads or preheat-type thermal heads. The present invention also generally relates to methods of making the thermal heads.
Furthermore, the invention generally relates to a lamp reflection board which reflects light from a lamp to thermal paper, and further relates to a small instrument such as the above mentioned color printer, in which it is very difficult to secure a sufficient optical path length.
2. Description of Related Art
Color Printer
There are many types of color printers, such as thermal transfer printers, ink-jet printers, and so on. Thermal transfer printers transfer ink from an ink ribbon onto recording paper by placing the ink ribbon against the recording paper, heating the thermal head and pressing it against the ink ribbon.
Recently, customers have required high speed printing from thermal transfer color printers. But the maximum printing speed is limited by four elements, i.e., as the characteristics of the ink ribbon dye, development of wrinkles, data transfer speed, and the response speed of the thermal head.
FIG. 1 illustrates a general block diagram of a conventional color printer. A conventional color printer has three thermal heads for three colors of ink ribbons, which are yellow, magenta, and cyan ribbons. The conventional color printer uses roll paper or cut paper of a long length for recording paper 3. This color printer 1 has four print portions 1a, 1b, 1c, and 1d. The first print portion la prints yellow ink onto the recording paper 3. The second print portion 1b prints magenta ink onto the recording paper 3. The third print portion 1c prints cyan ink onto the recording paper 3. The fourth print portion 1d overcoats the recording paper 3 after yellow, magenta, and cyan ink are printed onto the recording paper 3.
The recording paper 3 is conveyed from paper roll 2 by feed roller 4 and pinch roller 5. The feed roller 4 has a small protuberance on its surface, and is driven by a pulse motor (not shown in FIG. 1). Downstream of the fourth print portion id, there is a paper discharging roller 6 for discharging the recording paper 3, and cutter 8 for cutting the recording paper 3 to a pre-determined length.
Each print portion 1a, 1b, 1c has thermal heads 9a, 9b, 9c, platen rollers 10a, 10b, 10c, ink ribbon supply rollers 12a, 12b, 12c, ink ribbon take-up rollers 13a, 13b, 13c, tension rollers 14a, 14b, 14c, and pinch rollers 15a, 15b, 15c. The platen rollers 10a, 10b, 10c are located opposite the print portions 1a, 1b, 1c, respectively. The ink ribbon supply rollers 12a, 12b, 12c supply each color of ink ribbon 11a, 11b, 11c to the respective thermal heads 9a, 9b, 9c. The ink ribbon take-up rollers 13a, 13b, 13c respectively take up each color of ink ribbon 11a, 11b, 11c. The tension rollers 14a, 14b, 14c are located downstream of the respective thermal heads 9a, 9b, 9c. The pinch rollers 15a, 15b, 15c are located opposite the tension rollers 14a, 14b, 14c, respectively.
Similar to each print portion 1a, 1b, 1c, the overcoat portion 1d has thermal head 9d, platen roller 10d, overcoat ribbon 11d, overcoat ribbon supply roller 12d, overcoat ribbon take-up roller 13d, tension roller 14d, and pinch roller 15d. The overcoat ribbon supply roller 12d supplies overcoat ribbon 11d to the thermal head 9d. The overcoat ribbon take-up roller 13d takes up the overcoat ribbon 11d. The tension roller 14d is located downstream of the thermal head 9d. The pinch roller 15d is located opposite the tension roller 14d. 
F1, F2, F3, and F4 indicate the thrust tension of each of tension rollers 14a, 14b, 14c, and 14d. W1, W2, W3, and W4 indicate the frictional forces of each thermal transfer mechanism. T0 indicates the tension of the feed roller 4, and T1, T2, T3, and T4 indicate the tension of the paper 3 between the tension rollers 14a, 14b, 14c, 14d and the thermal heads 9a, 9b, 9c, 9d at the print portions 1a, 1b, 1c and overcoat portion 1d. 
At the above mentioned color printer 1, the front edge of paper 3 pulled out from paper roll 2 is conveyed between feed roller 4 and pinch roller 5 by a paper feeding mechanism (not shown in FIG. 1). Then the front edge of paper 3 is caught between feed roller 4 and pinch roller 5, passes between the thermal head 9a and platen roller 10a, and is conveyed to a location between tension roller 14a and pinch roller 15a. 
At this point, the thermal head 9a is pressed against platen roller 10a sandwiching the paper 3, and pinch roller 16a is pressed against tension roller 14a sandwiching the paper 3. Then, tension roller 14a is driven by a DC motor (not shown in FIG. 1) with constant tension, and at the same time yellow ink is transferred onto paper 3 by the heat of the thermal head 9a. Yellow ink ribbon 11a wound around the ink ribbon supply roller 12a is moved downstream of the printer in synchronization with the printing speed. The ink ribbon take-up roller 13a takes up the transferred ink ribbon 11a. 
After the yellow image printing is over, the front edge of paper 3 reaches a location between tension roller 14a and pinch roller 15a. At this point, the thermal head 9b is pressed into platen roller 10b sandwiching the paper 3, and pinch roller 15b is pressed into tension roller 14b sandwiching the paper 3. Then, tension roller 14b is driven by a DC motor (not shown in FIG. 1) with constant tension, and at the same time magenta ink is transferred onto paper 3 by the heat of the thermal head 9b. Magenta ink ribbon 11b wound around the ink ribbon supply roller 12b is moved downstream of the printer in synchronization with the printing speed. The ink ribbon take-up roller 13b takes up the transferred ink ribbon 11b. When a first magenta image is printed on a first portion of the paper 3 at the thermal head 9b, a second yellow image is printed on a second portion of the paper 3 at the thermal head 9a. 
After the magenta image printing is over, the front edge of paper 3 reaches the location between tension roller 14c and pinch roller 15c. At this point, tension roller 14c is driven by a DC motor (not shown in FIG. 1) with constant tension, and at the same time cyan ink is transferred onto paper 3 by the heat of the thermal head 9c. Then the front edge of paper 3 reaches the location between tension roller 14d and pinch roller 15d. When the first cyan image is printed on first portion of the paper 3 at the thermal head 9c, the second magenta image is printed on a second portion of the paper 3 at the thermal head 9b, and the third yellow image is printed on a third portion of the paper 3 at the thermal head 9a. 
When the front edge of paper 3 reaches the location between tension roller 14d and pinch roller 15d, the thermal head 9d is pressed against platen roller 10d sandwiching the paper 3, and pinch roller 15d is pressed against tension roller 14d, sandwiching the paper 3. Then, tension roller 14d is driven by a DC motor (not shown in FIG. 1) with constant tension, and at the same time an overcoat print is executed with heat of the thermal head 9d on paper 3, which has been printed with a yellow image, magenta image, and cyan image. And overcoat ribbon lid wound around overcoat ribbon supply roller 12d is moved downstream of the printer in synchronization with the printing speed. The overcoat ribbon take-up roller 13d takes up the transferred overcoat ribbon 11d. 
When overcoat is printed on a first portion of the paper 3 at the thermal head 9d, the second cyan image is printed on a second portion of the paper 3 at the thermal head 9c, the third magenta image is printed on a third portion of the paper 3 at the thermal head 9b, and the fourth yellow image is printed on a fourth portion of the paper 3 at the thermal head 9a. After overcoat printing is over, the front edge of paper 3 reaches cutter 8, passing through paper discharging roller 6 and pinch roller 7. The cutter 8 cuts the first portion of the paper 3 and a paper storage area (not shown in FIG. 1) stores the first portion of the paper 3, which is printed with a yellow image, magenta image, and cyan image and with an overcoat.
The above mentioned color printer and paper feeding have the following requirements. The first requirement is stability of the tension of the paper 3 during printing. Generally, the thrust tension F1, F2, F3, and F4 of each tension roller 14a, 14b, 14c, and 14d are made constant. Therefore, the tension affecting paper 3 and working on feed roller 4 changes incrementally from the first paper printing portion to the third paper printing portion. FIG. 2 illustrates the relation between the tension and the extension of the paper, which is used for sublimation printing with a 230 xcexcm thickness. The paper is distorted elastically by tension, and this elastic distortion strongly depends on humidity. As shown in FIG. 2, if the tension acting on the paper 3 changes during the printing operation, the change of tension causes paper expansion and contraction, producing shear of the colors in printing.
The second requirement is stability of the conveyance ratio. As shown in FIG. 1, the critical roller to determine conveyance length of paper 3 is feed roller 4 at the paper feeding mechanism. If the tension acting on the paper by feed roller 4 is changed, then the conveyance ratio of paper 3 changes. FIG. 3 shows feed roller 4 and pinch roller 5 feeding paper 3. As shown in FIG. 3, the conveyance ratio is expressed by the following equation, when xe2x80x9cRxe2x80x9d represents the radius of feed roller 4:
conveyance ratio (%)=(measured conveyance length/2xcfx80R)xc3x97100xe2x80x83xe2x80x83(1)
FIG. 4 illustrates the relation between the tension and conveyance ratio. In FIG. 4, a positive sign indicates that the direction of the tension acting on the paper is the same direction as the paper feeding, and a negative sign indicates that the direction of tension acting on the paper is the opposite direction to the direction of paper feeding. If the tension acting on the feed roller 4 changes during printing, the conveyance length of paper 3 changes and that leads to shear of the colors in printing.
To summarize above arguments of the color printer, the changes of the tension acting on the paper cause tension and extension of the paper; as well as a change of the tension acting on the feed roller 4 during continuous printing of a first portion of the paper to a third portion of the paper. Changes in tension acting on the paper result in changes of the conveyance length of the paper, which lead to shear of the colors in printing during the placing of color dots. Next to a third portion of the paper, the shear of the colors in printing is relatively small, because changes in tension occur within a limited range. To avoid shear of the colors for the first three portions of the paper, one conventional solution is to discard the first three portions of the paper without color printing. However, this method causes much waste of paper 3.
The third requirement is to avoid lateral print shading. In the case of a color printer using the thermal transfer method or thermal color development method, the frictional coefficient between the exothermic portion of the thermal head and the paper or ink ribbon fluctuates depending on the energy acting on the exothermic portion and the quantity of heat stored in the thermal head. This fluctuation occurs in each line of printing, and occurs on a millisecond order period in the time domain. This fluctuation of tension on the millisecond order leads to lateral print shading on recording paper 3, which reduces the printing quality. Furthermore, as shown in FIG. 4, if the tension acting feed roller 4 fluctuates during printing, this fluctuation changes the conveyance length of paper 3, which cause shear of the colors in printing.
Thermal Head
Japanese Unexamined Patent Publication, First Publication No. Sho 62-217627 discloses the invention of double line thermal heads, which have plural exothermic resistance portions with two lines in parallel to speed up printing. Double line thermal heads can cut the time for printing in half, in principle, because these thermal heads print two lines at the same time. Japanese Unexamined Patent Publication, First Publication No. Hei 08-300695 discloses an invention of a preheat-type thermal head with a metal plate, which speeds up printing.
However there are some problems with double line thermal heads. The first problem is peeling off at the boundary between the common electrode and alumina base when heating, because the thermal expansion coefficients of the bulk metal of the common electrode and of the alumina base are not the same. The second problem is that peeling off at the boundary of the electrode and alumina base causes thermal stress to the thin-film electrode, which builds up on the common electrode, and the thermal stress can damage the thin film, which has portions with low mechanical strength. Furthermore, peeling off at the boundary of the electrode and alumina base makes it difficult to smoothly connect the common electrode and the alumina base, and the thermal stress breaks the thin film formed on the common electrode.
The third problem is the difficulty in manufacturing a common electrode, which should have positioning accuracy within fine width of the dot level of the thermal head. It is very difficult to connect the common electrode with the alumina base without gaps or openings, and it is very difficult to cover the alumina base with the common electrode without gaps or openings. Even if these problems were overcome, a fourth problem occurs in that is difficult to print with high density with double line thermal heads, because there is the opening between the double lines in exothermic resistance portion, and the width of the opening is coincident to the width of the common electrode.
There are some problems with preheat type thermal heads. The first problem is that it is difficult to complete a predetermined shape precisely for all lengths of substrate in the process of producing a common electrode on a stainless substrate with etching or mechanical processing, with increases the cost. The second problem is that air bubbles remain in the glass glaze. In the process of screen-printing and baking of the glass paste, it is difficult to bake a stainless substrate with a baking temperature as high as that of the ceramic substrate. As the result of the relatively low baking temperature, the viscosity of the glass glaze remains high, and high viscosity hinders elimination of air bubbles in the glass glaze. The third problem is damage to the thin film formed on the common electrode, because air bubbles appear on the surface of common electrode through lapping or polishing of the preheat type thermal head. The fourth problem is that it is difficult to complete the preheat type thermal head with precise lapping or polishing over the entire length of the substrate. The lapping or polishing of the thermal heads is carried out to make the common electrode appear on the surface of the thermal heads.
Even if these problems were solved, a fifth problem would remain, which is a large loss of heat at the common electrode while the paper passes through the first line exothermic resistance portion to reach the second line exothermic resistance portion. The reason for the fifth problem is that the common electrode is located between the double lines of the exothermic resistance portion in the case of a preheat type thermal head.
Lamp Reflex Board
Lamp reflex boards are attached thermal recording devices such as color printers. FIG. 5 illustrates the principal components of a thermal recording device having lamp reflex boards. Paper 3 (TA paper) is discharged by feed roller 4 and pinch roller 5 from paper roll 2. The paper 3 is printed with the three primary colors, that is a yellow image is printed by yellow print module 1a and yellow fixing lamp 30a, a magenta image is printed by magenta print module 1b and magenta fixing lamp 30b, and a cyan image is printed by cyan print module 1c. Color printed paper 3 is then sent to cutter 8 by paper discharging roller 6 and pinch roller 7, and is cut to a predetermined length by cutter 8 driven by cutter motor 28. Yellow fixing lamp 30a and magenta fixing lamp 30b have yellow lamp reflex board 40a and magenta lamp reflex board 40b respectively, and these lamp reflex boards 40a, 40b make the light emanating from the lamps effectively irradiate the thermal paper 3.
FIG. 6 is a representative diagram of the prior art of lamp reflex board and its catoptric light incident on the thermal paper. The lamp reflex board shown in FIG. 6 is disclosed in Japanese Unexamined Patent Publication, First Publication Nos. Hei 09-216390 and 09-216391 for example. The following are the predetermined conditions for the lamp reflex board. Lamp 30 has a circular section with diameter of 16 mm. A reflex board 40 is attached having seven portions which are symmetrical about the lamp 30. The numbers of the lamp light beams illustrated in FIG. 6 is seventy-one, which are divided by seventy equal angles over a range of 110 degrees.
FIG. 6 shows the positions of connecting points P0, P1, P2, . . . , P7. According to FIG. 6, catoptric light reaches paper 3 with one or two reflections. After the shape of the reflex board 40 is determined in the above mentioned manner, the reflex board 40 is manufactured according to the determined shape. The width W* of the reflex board 40 is the distance between connecting point P7 and connecting point P7*, and it is expressed by following equation based on the diameter D of lamp 30.
W*=3.11Dxe2x80x83xe2x80x83(2)
The height H* of reflex board 40 from the surface of paper 3 is expressed by following equation.
H*=1.51Dxe2x80x83xe2x80x83(3)
It is desirable for the thermal recording device to be small, to make it easy to secure a place for the thermal recording device. Therefor it is desirable to make the width W* of the reflex board 40 narrow. According to the structure of the thermal recording device in FIG. 6, it is necessary to secure a width W* of reflex board 40 to make light radiated from the back of lamp 30 to reach the surface of the paper with one or two reflections. This makes it difficult to reduce the size of the thermal recording device, because a determined length of the reflection path of the light radiated from the lamp 30 must be provided.
It is the first object of the present invention to provide a color printer and a method of paper feeding which can prevent shear of the dots of each color arising from stretching of the paper by maintaining the tension acting on the paper constant to within a tolerance level. And it is also the first object of the present invention to provide a color printer and a method of paper feeding which can prevent shear of the dots of each color arising from irregular feeding by maintaining the paper feeding distance constant, which maintains the tension acting on the feed roller constant to within a tolerance level.
It is the second object of the present invention to provide a thermal transfer method or thermal color development method for a color printer which is provides high quality printed image by preventing lateral print shading of the printed image.
It is the third object of the present invention to provide a thermal head and a method of making the same, which is made by a simpler manufacturing process compared with conventional process to manufacture double-line thermal heads and preheat thermal heads.
It is the fourth object of the present invention to provide a thermal head which can print with higher density compared with conventional double-line thermal heads.
It is the fifth object of the present invention to provide a thermal head which has higher heat efficiency compared with conventional preheat thermal heads.
It is the sixth object of the present invention to provide a lamp reflex board, which has small width and which can make the light radiated by the back of the lamp reach the surface of the paper with two or one reflections.
The present invention of a method of paper feeding in a color printer provides an advantageous feature of achieving the first object. In the present invention, the recording paper is conveyed by a feed roller to plural recording portions sequentially, and each recording portion successively prints each specific color on the recording paper while applying tension to the paper. The tension applied to the recording paper is switched to a predetermined value each time the recording paper is conveyed to each recording portion when starting to print.
Therefore the present invention of a method of paper feeding in a color printer can provide a tension applied to the recording paper which is switched to a predetermined value each time the front edge of the recording paper is conveyed to each recording portion when starting to print. For example, when the front edge of the recording paper reaches the first recording portion, a tension is applied to the recording paper in excess of the frictional force experienced by the recording paper at the first recording portion. This provides a tension corresponding to the frictional force experienced by the recording paper at the first recording portion, and a further tension in excess of the frictional force of the feed roller.
When the front edge of the recording paper reaches the second recording portion, the tension at the second recording portion is set to be in excess of the frictional force of the recording paper in the same manner as described above for the first recording portion. At the same time, the tension at the first recording portion is set to correspond to the frictional force on the recording paper. This makes the tension correspond to the frictional force on the recording paper at the first and second recording portions, and provides a tension in excess of the frictional force of the feed roller between the second recording portion and the first recording portion.
The present invention of a color printer provides an advantageous feature of achieving the first object. In the present invention, the color printer comprises a feed roller, plural recording portions, and tension rollers. The feed roller conveys the recording paper. The plural recording portions are located along the paper path downstream of the feed roller, and each prints a specific color ink on the paper. The tension rollers are located downstream of each recording portion, and convey the recording paper from upstream to downstream by applying tension. Furthermore, the tension that the tension rollers apply to the recording paper is switched to a predetermined value each time the recording paper is conveyed to each recording portion.
Therefore the present invention of a color printer can provide a tension applied to the recording paper which is switched to a predetermined value each time the front edge of the recording paper is conveyed to each recording portion when starting to print. For example, when the front edge of the recording paper reaches the recording portion, tension is applied to the recording paper in excess of the frictional force of the recording paper at the recording portion. This makes the tension correspond to the frictional force on the recording paper at the recording portion, and provides a further tension in excess of the frictional force of the feed roller.
The present invention of a color printer also provides the advantageous feature of achieving the second object. In the present invention, the color printer comprises a feed roller, plural recording portions, tension rollers, a frictional force detecting portion, and a driving force control portion. The feed roller conveys the recording paper. The plural recording portions are located along the paper path downstream of the feed roller, and each prints a specific color ink on the paper. The tension rollers are located downstream of each recording portion, and convey the recording paper from upstream to downstream by applying tension. The frictional force-detecting portion detects a frictional force between the thermal head located on the recording portion and the paper during printing. The driving force control portion controls the driving force of the motor to balance the frictional force detected by the frictional force detecting portion and the driving force of the motor driving the tension roller.
The present invention of a thermal head provides the advantageous feature of achieving the third, fourth, and fifth objects. In the present invention, the thermal head comprises a first layer of an electric circuit pattern, an insulator layer, and a second layer of an electric circuit pattern. The first layer of the electric circuit pattern has an electric circuit pattern formed on a ceramic substrate. The insulator layer is formed on the layer of the electric circuit pattern. The second layer of the electric circuit pattern has an electric circuit pattern formed on the insulator layer.
The present invention of a lamp reflex board provides an advantageous feature of achieving the sixth object. In the present invention, the lamp reflex board reflects radiant light from the back and side of the lamp onto an irradiation surface, and comprises a rear reflex board, a rear-side reflex board, and a side reflex board. The rear reflex board is located at the back of the lamp, and it reflects radiant light from the back of the lamp to the side of the lamp. The rear-side reflex board is located to the back and side of the lamp, and it reflects catoptric lights from the rear reflex board onto the irradiation surface. The side reflex board is located to the side of the lamp, and it reflects radiant light from the side of the lamp onto the irradiation surface.
The rear reflex board and rear-side reflex board reflect radiant light from the back of the lamp onto the irradiation surface and the side reflex board reflects radiant light from the side of the lamp to the irradiation surface. Compared with a conventional reflector of a concave mirror, the rear reflex board, rear-side reflex board, and side reflex board are provided as separate pieces. This makes the width of lamp reflex board narrow.